The present invention relates to a solder paste for the mounting of electronic components and substrates that contains a mixture of oxalic acid, adipic acid, and an amine component, as well as to a method for the production thereof. Moreover, the present invention relates to a method for the mounting of an electronic component on a substrate through the use of the solder paste according to the present invention, as well as to a module that comprises the solder paste according to the present invention.
The main application of solder pastes, in particular of soft solder pastes, is in the production of electronic circuits, where they serve mainly as electrical and thermal connections between a component and a substrate, for example between a semiconductor chip and a printed circuit board (PCB). The solder paste is being applied to the printed circuit board in this context. The semiconductor chip is then contacted to the printed circuit board via the solder paste. The solder paste is heated up in order to melt the solder in the paste by means of a reflow process by means of which electrical and/or thermal contacts are formed between the semiconductor chip and the printed circuit board. The solder pastes usually contain soldering fluxes that serve, inter alia, to dissolve the oxide layer on the surface of the soldering powder, components, and substrate and to thus provide for improved wettability during the soldering process. Since solder pastes are usually processed in the automatic production of electronic circuits by means of screen and stencil printing, the soldering fluxes usually contain additional substances, such as surfactants and additives, that improve the rheological properties of the soldering fluxes, and thus of the solder paste.
DE 10 2012 019 198 A1 describes a flux composition comprising a carboxylic acid and a polyamine flux, whereby the polyamine comprises non-substituted C5-80 alkyl groups with at least two tertiary carbon atoms.
EP 0 619 162 A2 discloses a soldering flux comprising a 2-bisoxazoline compound, a dithiol, an organic carboxylic acid, and an activator. Moreover, the soldering flux can comprise an organic solvent, a thermoplastic resin and/or an epoxy group-containing compound.
DE 42 35 575 C2 describes a soft solder paste for soldering electronic components and circuits in the presence of a protective gas, in particular in the presence of nitrogen with a low residual oxygen content. In this context, the soft solder paste consists of an organic resin, an activator, and a solvent.
DE 41 19 012 C1 relates to a solder paste for electronics components that can be rinsed with water and contains water-soluble salts of fatty amines and organic acids as soldering flux mixture.
WO 2010/000679 A1 relates to soldering materials containing metal stearates. In this context, the particles of the soldering powder comprise a fine layer of solid metal stearate on the surface.
U.S. Pat. Application Publication No. 2009/0152331 A1 describes a resin-free solder paste comprising a metal powder, a polar solvent, a carboxylic acid, and a tertiary amine. The paste is particularly well-suited for under-filling (under-bumper metallization) as well as for mounting of surface-mounted components (surface-mounted technology).
One special challenge of the production of electronic circuits is the application and the mounting of components that comprise not only electrical connecting sites, but also thermal connecting sites on the underside of the component, such as, for example, QFPs with contacts to corresponding heat sinks in the substrate. The electrical contact sites, so-called “legs”, serve also as spacer between the component and the substrate and are contacted by means of corresponding solder depots in the course of the reflow process. The thermal connecting sites are also connected to the substrate by means of a reflow process using solder depots, whereby the solder pastes used for thermal contacting can be different from those that are used for electrical contacting of the component to the substrate. In the course of this process, the introduced solder depot of the thermal connecting sites might spread upon heating. In this context, solder powder particles are driven out in the direction of the electrical contacting sites, usually in the form of so-called soldering balls, and might cause short-circuiting at these sites.
To prevent this from occurring, there is a need to have solder pastes that do not spread when heated and in which undesired solder balls are prevented from arising at the electrical contact sites and short-circuiting is thus prevented, while, at the same time, a stable mechanical, thermal and/or electronic connection between the component and the substrate is being generated. Moreover, there is a need for solder pastes that form clear wetting edges when melted and thus permit precise printing on substrates and thus permit the production of small integral circuits.